A CAN (Controller Area Network) has been conventionally used, for example, as a communication protocol of a LAN (local area network) in an automobile.
Here, an outline of the CAN will be described by the use of FIG. 7. In this respect, in FIG. 7 and the following description based on FIG. 7, it is assumed that each communications device conducting communication in accordance with the CAN protocol is one of a plurality of (=n) electronic control units (hereinafter referred to as ECU) 101 for controlling the respective parts of an automobile. Moreover, in the following description, in a case of particularly differentiating the ECUs 101 from each other, they are referred to as ECU (1), ECU (2), ECU (3), . . . , and ECU (n) from the ECU 101 on the extreme left to the right in FIG. 7.
As shown in FIG. 7, in the CAN, a two-wire communications line 13 including a CAN-H line (hereinafter simply referred to as “H line”) 11 and a CAN-L line (hereinafter simply referred to as “L line”) 12 is used as a communications line (so-called communications bus) and terminating resistors R are connected to both ends of the two-wire communications line 13. Here, the resistance of the terminating resistor R is 120 Ω, for example. Moreover, in the embodiment shown in FIG. 7, the terminating resistors R are provided in the two ECU (1) and ECU (n) arranged on the ends of the two-wire communications line 13, respectively.
Then, in the CAN, the communications device (ECU 101) for sending data sends an inverse signal to the H line 11 and the L line 12, and the communications device for receiving the data determines whether data on the communications line 13 is “1” or “0” from a difference in voltage between the H line 11 and the L line 12. As a specific configurational example, each ECU 101 is provided with a CPU 21 and a communications driver 23. The CPU 21 executes a control processing for controlling the respective parts of the automobile and a processing for conducting communication with the other ECUs 101. The communications driver 23 is connected to the foregoing two-wire communications line 13 and outputs data given and sent by the CPU 21 to the two-wire communications line 13 and inputs data on the two-wire communications line 13 to the CPU 21. Then, the communications driver 23 is provided with an output buffer 31, an output buffer 32, and a binarization circuit 33. The output buffer 31 makes the voltage of the H line 11 a high level (for example, 5 V) when the data to be sent is “0,” and makes the voltage of the H line 11 a low level (for example, 2.5 V) when the data to be sent is “1.” The output buffer 32 makes the voltage of the L line 12 a low level (for example, 0 V) when the data to be sent is “0” and makes the voltage of the L line 12 a high level (for example, 2.5 V) when the data to be sent is “1.” The binarization circuit 33 has the voltage of the H line 11 and the voltage of the L line inputted thereto to generate a binary signal of “1” or “0” expressing data (in turn, data to be received) on the communications line from the difference between both the voltages. Further, if the two-wire communications line 13 is normal in this CAN, the respective ECUs 101 can conduct communication at as high a communication speed as 500 kbps, for example.
Still further, in a case where a failure (break failure, or short-circuit failure) occurs in the H line 11 or the L line 12, each ECU 101 can conduct communication, even though at a communication speed (for example, 125 kbps) lower than the high communication speed of 500 kbps described above, by the use of only the normal line (H line 11 or L line 12). In other words, even when either the H line 11 or the L line 12 breaks down, each ECU 101 can conduct communication. In this case, however, each ECU 101 uses only one of the H line 11 and the L line 12 and thus is susceptible to noises, fluctuations in signal level, and the like, whereby the communication speed is limited to a low value.
By the way, in the communications system shown in FIG. 7 in accordance with the forgoing CAN, it is assumed that the respective ECUs 101 send or receive data among them through the high-speed communication of 500 kbps to control the respective parts of the automobile.
In this case, when a break failure occurs at any point in the two-wire communications line 13 (for example, between ECU (1) and ECU (3)), there is brought about a state in which the two-wire communications line 13 is not provided with terminating resistors R. Thus, this makes it impossible for all the ECUs 101 to conduct communication at an intrinsic communication speed (that is, the foregoing high-speed communication of 500 kbps under normal operating conditions) and hence to properly conduct a processing to be executed (control of the automobile, in this example).
In this respect, for example, JP-A No. 165415/2000 discloses the following technique. All ECUs to be connected to a CAN bus (two-wire communications line of CAN) are previously provided with resistors that can be terminating resistors. The foregoing resistors of the ECUs arranged at the ends of the CAN bus are connected as the terminating resistors to the CAN bus by switches or the like. Here, wherever the ECU is connected to the CAN bus, it is possible to circumvent the need of mounting or dismounting the terminating resistor on or from the ECU or modifying the ECU. However, this technique disclosed in this publication of patent application can not produce a fail-safe effect in a case where a break failure occurs in the CAN bus and hence can not solve the foregoing problem.